Rinse the lentils a few times in water. Then transfer them to a 2 litre pressure cooker. Add ¾ to 1 cup water and turmeric powder. Pressure cook on medium heat for for 5 to 6 whistles or till the lentils have completely softened. If cooking in a pan, then soak the lentils for one hour. Drain all the water. Cook the lentils in a pan or pot adding required amount of water.
After the pressure cooking is complete, open the lid only after the pressure falls naturally in the cooker.
Once cooked and softened, mash the lentils very well with a spoon or wired whisk and keep it aside.
Preparation
In a mortar, take the cumin seeds, black pepper and garlic cloves.
Crush coarsely with the pestle. Set aside. You can also use a small spice grinder to crush these spices.
Chop pineapple into small cubes and set aside. You will need 1 cup of pineapple cubes.
Add half of the chopped pineapple cubes in a grinder jar or small blender.
Blend to a smooth puree. Take this puree in a bowl and keep aside.
To the same jar, add the roughly chopped tomato.
Pulse the jar a couple of times to crush the tomatoes. You don't need to make a puree. You can even crush the tomatoes with your hands.
Tempering and making pineapple rasam
Heat oil in a pot or pan and keep the flame to a low. Add the mustard seeds and let them crackle.
Now, add the coarsely crushed paste of garlic-black pepper-cumin and dried red chilies. Simply mix and stir once on a low heat so that the spices do not get burnt.
Quickly add the curry leaves and asafoetida.
Mix again and then add crushed tomatoes.
Sauté for 2 to 3 minutes on a low to medium heat till the raw aroma of tomatoes goes away. You should also be able to see oil leaving the sides of the tomato and spices mixture.
Add the pineapple puree. Stir and sauté for 2 minutes.
Then add the cooked and mashed lentils. Next, add the remaining half of the chopped pineapple cubes. Stir and mix again. Add 1.5 cups water.
Add rasam powder. This is optional. You can skip if you don't have rasam powder.
Season with salt as per taste. Stir very well and bring the rasam to a simmer on low to medium heat, without the lid for 8 to 9 minutes.
Garnish with chopped coriander leaves and serve Pineapple Rasam hot with steamed rice or as an appetizer soup.
Notes
You can also use moong dal instead of tur dal or a mix of both the lentils.
Make sure to make the rasam with pineapples that are sweet with a touch of tang. Do not make the rasam with pineapple that are very sweet or pineapples that are overly sour or unripe.
To make the dish gluten-free, skip adding asafoetida or use gluten free asafoetida.